Oxidative dehydrogenation of paraffinic hydrocarbons



United States Patent 3,270,086 OXIDATIVE DEHYDROGENATION 0F PAR- AFFINlC HYDROCARBONS Robert B. Regier, Bartlesville, 0kla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed July 29, 1965, Ser. No. 475,852 5 Claims. (Cl. 260-6833) This invention relates to a process for the oxidative dehydrogenation of paraffinic hydrocarbons. In another aspect, this invention relates to a process for the oxidative dehydrogenation of a paraflinic hydrocarbon in the presence of a molten salt medium.

Oxidative dehydrogenation is an attractive route for the production of olefins from paraffinic hydrocarbons. The oxygen by combining With the hydrogen to form Water has a favorable effect upon the equilibrium of the reaction mixture. However, conventionally required high temperature reactions involving hydrocarbon and air or oxygen present serious control problems. It is not always possible to accurately control the reaction and to minimize losses of the feed stock which form oxides of carbon or other undesirable by-products as the result of localized overheating.

Accordingly, an object of my invention is to provide a process for the production of olefins from paraffinic hydrocarbons.

Another object of my invention is to provide a relatively low temperature process for the oxidative dehydrogenation of paraffinic hydrocarbons to produce olefins.

Other objects, advantages and features of my invention will be readily apparent to those skilled in the art from the following description and appended claims.

I have by my invention provided a process for the production of olefins from paraflinic hydrocarbons wherein a paraffinic hydrocarbon feed is contacted with a molten inorganic salt medium in the presence of oxygen at a temperature in the range of about 400 to about 700 C.

Although I have found that the use of particular molten inorganic salt mediums valuable for the promotion of the desired oxidative dehydrogenation reaction and/or providing convenient temperature control of the oxidative dehydrogenation reaction, it must be understood that not every liquid molten inorganic salt medium is capable of being successfully employed in the oxidative dehydrogenation of paraifinic hydrocarbons. Many inorganic salt melt systems have been found to be antagonistic to the dehydrogenation reaction and the use of these particular inorganic salt medium results in drastically inhibiting the desired oxidative dehydrogenation reaction. For example, melt systems containing such molten inorganic compounds of Group I metals as cuprous chloride or silver oxide are capable of providing eflicient temperature control but have an inhibiting effect upon the oxidative dehydrogenation reaction. These particular materials have been found to reduce or essentially stop the dehydrogenation reaction.

The molten salt medium employed in the process of my invention comprises an alkali metal carbonate or a mixture of alkali metal carbonates. The individual carbonates are relatively high melting, the carbonates of lithium, sodium, potassium, rubidium and cesium having melting points of 618, 851, 891, 831 and 610 C., respectively. Temperatures above 700 C. are generally too high for oxidative dehydrogenation and tend to promote cracking of the parafiinic hydrocarbon feed. In those instances where the melting point of the carbonate is below 700 C., the individual carbonate can be employed in the process of my invention. However, mixtures of two or more of the carbonates are known to exhibit a generally depressed melting point which would permit them to be successfully employed as a molten "ice salt medium in oxidative dehydrogenation reactions conducted at as low as 400 C. For example, a 4:313 mol ratio of lithium, sodium and potassium carbonate, respectively, produces a mixture which melts at about 400 C. The invention is applicable to a single or a mixture of two or more alkali metal carbonates which produce at least partially molten mixtures in the temperature range of 400-700 C.

The parafiinic feed hydrocarbon-s which are applicable to the process of my invention are those parafiinic hydrocarbons having at least 4 carbon atoms per molecule and preferably having from 4-10 carbon atoms per molecule. These paraffinic hydrocarbons can be unbranched, branched, cyclic, acyclic or combinations thereof. Suitable paraffinic hydrocarbon feed materials include n-butane, isobutane, isopentane, cyclohexane, methylcyclohexane, 2,4-dimethylcyclohexane, decane, and the like.

The oxidative dehydrogenation reaction is conducted in the vapor phase and at a temperature in the range from about 400 to about 700 C. The pressure under which the oxidative dehydrogenation process is conducted can vary over a wide range, but will normally be in the range of 0-750 p.s.i.g.

The oxidative dehydrogenation reaction is conducted in the presence of air or other oxygen-containing gas. The ratio of parafiinic hydrocarbon feed to air or oxygen which is applicable to the present invention can vary widely but preferably excludes the explosive mixture range for the particular paraflinic hydrocarbon feed material. It is preferred to operate on the fuel-rich side of the explosive mixture range. For example, the explosive limits for n-butane and air are about 1.6-8.5 mol percent butane. Hence, a preferred ratio of n-butane to oxygen is 05-10 mols of n-butane per mol of oxygen (a 2:1 mol ratio is theoretically required to convert butane to l-butene and water). Still higher mol ratios of n-butane to oxygen are operable but result necessarily in lower conversion. It has been discovered that lower n-butane-oxygen ratios (greater quantities of air) up to the explosive limit range as previously indicated, offer greater conversion to olefins with little or no loss of selectivity. In operation, the molecular oxygen is normally completely converted and does not appear in the effluent.

Preferably, although not to be limited thereto, the paraflinic hydrocarbon feed is blended by conventional means with the required amount of oxygen-containing gas and the mixture preheated until it is at or near the reaction temperature. This homogeneous mixture is then contacted with the previously described molten salt medium within the reaction zone. It is also within the scope of this invention to introduce the oxygen-containing gas and parafiinic hydrocarbon feed independently into the oxidative dehydrogenation zone.

Although not to be limited thereto, the oxidative dehydrogenation reaction is preferably conducted continuously. The reaction mixture comprising paratfinic hydrocarbon feed and oxygen-containing gas is contacted with a molten salt medium in the reaction zone such that a residence time in the reaction Zone is normally about 0.5-30 seconds, preferably 1-15 seconds.

Although not to be limited thereto, a suggested method for effecting the oxidative dehydrogenation reaction comprises introducing a vaporous paraffinic hydrocar bon and oxygen-containing gas mixture to the bottom of a vertical column containing a selected molten salt me dium. The vaporous feed is permitted to travel upwardly through the column in the form of fine bubbles. The effluent gases leaving the top of the column containing the oxidative dehydrogenated product, water vapor, some oxides of carbon and unconverted feed material, can then be separated by conventional means. The olefin prodnets are recovered and unconverted paraflinic hydrocarbon can be recycled to the dehydrogenation zone.

The following example is presented as illustrative of the objects and advantages of my invention. However, it is not intended that the invention should be limited to the specific embodiments presented therein.

Example A inch O.D. stainless steel tube, closed at one end, was packed with sufllcient lithium carbonate, sodium carbonate and potassium carbonate (mol ratio 4:3:3, respectively), to prepare about a 9 inch column when molten. This stainless steel reactor was encased in a graphite tube which was in contact with an external heater. The salt mixture was heated to 450 C. A metered blend of n-butane and air, with the n-butane to oxygen mol ratio of 2.0, was passed into the molten salt by means of a stainless steel dip tube extending to the bottom of the salt filled reaction tube. The dip tube was inch OD. and about /8 inch ID. The mixture of nbutane and air was permitted to escape from the bottom of the dip tube and to bubble upwardly through the top of the molten salt column and out the top of the reaction tube at atmospheric pressure. The exit efiluent gas was passed through a condenser cooled to the ice point and was then periodically sampled for analysis. The residence time of the reaction mixture in the feed salt medium was between 1.0 and 1.5 seconds. The oxidative dehydrogenation reaction was conducted at several temperatures and the results presented in the table below:

Run No 1 2 3 4 Temperature, C 450 500 550 600 CH mol percent- 0.1 0.1 0. 1 1. 5 00, mol percent" 3. 2 2. 5 2.0 0.9 002, mol percent 2. 2 2. 6 3. 3 6. 6 02+ Cr, mol percent 0. 6 0.4 O. 5 1. 7 C3, mol percent 0.2 0.2 0.2 1. 4 n-O4, mol percent 93.0 94. 4 94. 6 92. 7 Butenes", mol percent- 5. 2 4.1 3.5 3.1 Conversion, percent 7.0 5.6 5. 4 7. 3 Selectivity to butenes* 74 73 65 43 *Some runs contain minor quantities of butadiene.

The above data demonstrate that butane is oxidative dehydrogenated with good selectivity to butene, particularly at the lower reaction temperatures.

As will be evident to those skilled in the art, various modifications of this invention can be made, or followed, in the light of the foregoing disclosure, without departing from the spirit or scope thereof.

I claim:

1. A process which comprises contacting a vaporous parafiinic hydrocarbon having at least 4 carbon atoms per molecule in a dehydrogenation zone with a molten salt medium in the presence of an oxygen-containing gas, said molten salt medium selected from the group consisting of lithium carbonate, cesium carbonate, and mixtures of at least two of the carbonates of lithium, sodium, potassium, rubidium and cesium, and maintaining the temperature in said dehydrogenation Zone in the range from about 400 to about 700 C.

2. The process of claim 1 wherein the oxygen-containing gas and paraflinic hydrocarbon feed to said dehydrogenation zone is premixed prior to introduction of the combined mixture into said dehydrogenation zone.

3. The process of claim 1 wherein said paraifinic hydrocarbon is n-butane.

4. The process of claim 3 wherein said molten salt medium comprises a mixture of lithium, sodium and potassium carbonates.

5. The process of claim 1 wherein the residence time in said dehydrogenation zone is in the range of 0.5-30 seconds.

References Cited by the Examiner UNITED STATES PATENTS 3,080,435 3/1963 Nager 260683.3

DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS, Assistant Examiner. 

1. A PROCESS WHICH COMPRISES CONTACTNG A VAPOROUS PARAFFINIC HYDROCARBON HAVING AT LAST 4 CARBON ATOMS PER MOLECULE IN A DEHYDROGENATION ZONE WITH A MOLTEN SALT MEDIUM IN THE PRESENCE OF AN OXYGEN-CONTAINING GAS, SAID MOLTEN SALT MEDIUM SELECTED FROM THE GROUP CONSISTING OF LITHIUM CARBONATE, CESIUM CARBONATE, AND MIXTURES OF AT LEAST TWO OF THE CARBONATES OF LITHIUM, SODIUM, POTASSIUM, RUBIDINIUM AND CESIUM, AND MAINTAINING THE TEMPERATURES IN SAD DEHYDROGENATION ZONE IN THE RANGE FROM ABOUT 400 TO ABOUT 700*C. 